1. Field of the Invention
The present invention relates to a solid-state image pickup apparatus for compensating for color shading on a picture picked up by a solid-state image sensor having photo-sensors arranged, each of which has a set of photosensitive cells different in sensitivity from each other, to produce image information over a broad dynamic range, and a method of compensating for color shading for the same.
2. Description of the Background Art
It is a common practice with a solid-state image pickup apparatus capable of picking up a picture over a broad dynamic range by a solid-state image sensor, which has photo-sensors arranged, each having a primary and a secondary photosensitive cell different in sensitivity from each other. The primary cell, higher in sensitivity to incident light than the secondary cell, forms an image with a well modulation. The secondary cell accurately captures image, or contrast, information even on an imaged region that would cause a white blur in a usual exposure condition or would cause the primary cell to produce a saturated signal. U.S. patent publication No. US 2003/0141564 A1 of Kondo et al, for example, proposes a solid-state image pickup device capable of picking up a picture over a broad dynamic range by processing signals outputted from the main and subsidiary photosensitive fields.
Generally, in a solid-state image pickup device, image signals outputted from a solid-state image sensor may involve shading due to, e.g. unevenness in quantity of light incident to the respective photo-sensors. For example, in the case of a digital camera including a solid-state image sensor which has microlenses formed, the quantity of incident light to the respective photo-sensors noticeably varies in dependence upon the direction of the incidence particular to the microlenses. More specifically, to a photosensitive cell located in the vicinity of the edge of the imaging frame of the image sensor, light is incident often inclined, so that the incident light is poorer in quantity than one incident to a photosensitive cell located near the center of the frame. Consequently, the luminance of the signals produced in the vicinity of the edge of the frame is poorer, thus resulting in shading.
Japanese patent laid-open publication No. 79773/1996 discloses a shading correcting device for correcting shading by multiplying image signals outputted from an image sensor by a shading correction coefficient, which is calculated by approximating by a quadric curved surface function. The shading correcting device with this configuration can accomplish a shading correction feasible for mass-production of image sensors and therefore free the image sensors from irregularity.
Further, U.S. patent publication No. US 2002/0008760 A1 of Nakamura teaches a digital camera for correcting shading on the basis a pixel-by-pixel correction value. The digital camera divides an imaging area into a preselected number of blocks and stores light amount correction data particularly assigned to the blocks beforehand. When correcting shading, the digital camera calculates the correction value of each pixel by weighting the light amount correction data in accordance with the positions of target pixels to thereby generate correction values for respective pixels.
The prior art documents described above have the following problems left unsolved. In Kondo et al, the solid-state image pickup device picks up an image over a broad dynamic range by processing signals outputted from the main and subsidiary photosensitive fields each constituting a particular pixel. However, the problem with Kondo et al is that all subsidiary fields in an imaging frame are arranged at one side with respect to the main photosensitive field without regard to the pixel position in the imaging frame, thus involving shading on a picture picked up which depends upon, e.g. the exit pupil position of a lens or an iris value of the camera.
The shading correcting device disclosed in Japanese publication No. 79773/1996 needs to pick up, during adjustment, a subject radiated with uniformly light, and, on the basis of the result from adjustment, the shading correction gain is determined. Then, in the adjustment procedure, correction data have to be obtained in accordance with, e.g. a zoom lens position or an iris value, and therefore, a long period of time is necessary for calculation. Although the shading correction of such a shading correcting device may be desirable for manufacturing solid-state image sensors, it is not feasible for adjusting or calibrating digital cameras.
Nakamura has a problem that because a memory in the digital camera stores correction data in one-to-one correspondence to various zoom lens positions and various iris values, the memory is required to have its storage capacity increased. Should the number of zoom lens positional sections and that of iris value sections be reduced in order to reduce the data amount required to be stored in the memory, the digital camera would involve image pickup conditions in which a complete correction is unable.
Apart from the problems stated above, secondary image data outputted from the secondary cell are used to reproduce high-luminance information and therefore often remain contained even in a completed picture in the form of high-luminance information. On a picture containing such high-luminance information, luminance shading, common to R (red), G (green) and B (blue) pixels, is not conspicuous. RGB or coloristic shading is dependent upon the wavelength of the colors R, G and B. Consequently, the coloristic shading is different in the degree of deviation between colors so as to be deviate much more toward the longer wavelength. This causes a picture to be viewed worse and critically degraded in image quality.